Large commercial transport aircraft have traditionally utilised hot-air ice protection systems. The hot-air is supplied via bleed from the engines. Commonly hot-air systems do not employ temperature monitoring equipment. Electro-thermal heater systems previously have been widely used to provide ice protection to areas such as engine inlets and helicopter rotors. It has been proposed to use electrically powered heater mats integrated into the wing structure to provide electro-thermal ice protection in the Boeing 787 Airliner. Temperature monitoring of such heater mats is necessary.
Wing structures have been traditionally of metallic construction but there is an increasing trend towards the use of composite materials. Electro-thermal heater mats are either bonded onto a metallic structure or are built directly into a composite structure. Composite structures typically comprise many individual layers of differing materials and properties. It is vital to monitor the temperature of structures incorporating electro-thermal ice protection systems to ensure overheat does not occur leading to potential structural failure, particularly in composite structures. In current systems, this is performed by embedding discrete sensors (e.g. thermo-couples, etc.) into the composite structure during manufacture or bonding them onto the surface of the structure.
Integration of discrete sensors into composite structures may cause many problems during the manufacturing process. Ensuring they are located at the correct location throughout the lay-up and cure process is problematic. Their physical size can compromise the structural integrity and their lack of robustness is such that there are often significant numbers of inoperable sensors after the cure process. The electrical connection to the sensors can be problematic with the added issue of electro-magnetic compatibility and interference. Additionally, during test and development (either full-scale or wind-tunnel) many more temperature sensing points are required to validate the design, which compound these problems. Given the problems associated with integrating each discrete sensor, known systems generally only monitor the structural temperature at what is perceived to be the single most critical location. However other locations, where temperature-related problems may occur, may be significant.